It is frequently necessary or desirable to selectively couple and uncouple an operator from a device. A particular application of this coupling need is in hydraulic machines such as skid-steer loaders and other hydraulic devices which may have other functions, such as but not limited to, street sweepers, ground boring machines, bull dozers, graders and earth scoopers. It is necessary to enable hydraulic systems, such as hydraulic lifts, only when a person is seated on the machine. In the past, this has been done by a solenoid operated lock pin which prevents operation of a valve spool by engaging a groove in the spool to retain the spool in a neutral position. Due to tolerance stack-ups, if an operating lever is pushed hard enough, the device can receive a small amount of oil and creep instead of remaining in position. Moreover, this type of solenoid spool lock requires that the operating handle be returned to neutral before the lock pin re-engages the groove in the spool. This leads to dangerous conditions in which the hydraulic device moves or operates when the lock pin should be engaged. A primary drawback of the solenoid projected lock pin is that the operating handle is always positively connected to the spool and will apply a longitudinal force to the spool whenever the actuating lever is pushed or pulled. It is therefore necessary to rely on the interference of the lock pin in order to prevent operation of the hydraulic device.
Hydraulic machines are exemplary of devices having safety features which disabled the machines under certain circumstances. There are numerous other devices which employ safety devices that have drawbacks which may be similar, analogous or in addition to the afore-discussed drawbacks of solenoid operated locking pins.
The difficulties are especially acute when it is necessary to prevent the operation of a device which moves in both directions, i.e., a device which is both pushed and pulled during its operation.
In view of these and other considerations, there is a need for improving the reliability and safety of devices which utilize push-pull operators.